Mask blanks used in the semiconductor design rule 1× generation (half-pitch (hp) 14 nm, 10 nm, etc.) include a reflective mask blank for EUV exposure, a binary mask blank for ArF excimer laser exposure, and a phase shift mask blank.
High flatness is required for a main surface of a mask blank substrate (that is, a surface on which a transfer pattern is to be formed) used to manufacture the reflective mask blank for EUV exposure used in the semiconductor design rule 1× generation. In order to meet the requirement for flatness, double-sided polishing for polishing both the front and back surfaces of the mask blank substrate using a polishing pad such as a polishing cloth and a polishing solution containing polishing abrasive grains has been employed often, as disclosed in, e.g., Patent Document 1. However, regarding the polishing of the transparent substrate by a conventional double-side polishing device, there is a limitation in increasing flatness of the main surface. Thus, the technique for flattening the main surface by measuring a shape of the main surface of the substrate and applying the plasma etching on a relatively convex portion has been developed, as shown in Patent Document 2.
Regarding exposure apparatuses having ArF excimer lasers as light sources, the numerical aperture (NA) has been increased, and the focus tolerance in the lithography step has been decreased. This has given rise to a problem of a profound effect on transfer accuracy from a wavefront aberration of a projection optical system. As measures to solve this problem, Patent Document 3 applies a correction optical element to a projection optical system for reducing the wavefront aberration. This correction optical element has a surface shape which can be defined by a Zernike polynomial for correcting an aberration resulting from a lens heating effect of the projection optical system. On the other hand, in EUV lithography comprised of an optical system comprised of a plurality of mirrors, the wavefront correction by controlling a deformable mirror has been considered, as disclosed in Patent Document 4.
As discussed below, Patent Document 5 discloses a device which can measure both the surface shape and the distribution of thickness variation for both the front and back main surfaces of the substrate.